The dual slope technique for determining the value of an unknown voltage involves charging a capacitor using an unknown but constant current derived from the unknown voltage for a fixed time and then measuring the time it takes to discharge the capacitor using a current derived from a known voltage. The ratio of the two times is a measure of the unknown voltage. Ammann in U.S. Pat. No. 3,316,547 discloses an integrating analog-to-digital converter using a dual slope technique as does Roswell Gilbert in U.S. Pat. No. 3,051,939. Propster in an article entitled "Analog to Digital Converter" published in IBM Technical Disclosure Bulletin Vol. 5, No. 8 dated January 1963 also discloses a version of the dual slope technique for measuring the value of an unknown voltage. Unfortunately, constant current sources necessary for the prior art dual slope voltage measurement structures and methods are quite expensive, particularly if the current source is linear over a range of voltages.